1. Field of the Invention
This invention relates to thick film capacitors and, in particular, to a high dielectric constant material for making such capacitors.
2. Background of the Invention
In the semiconductor art, the use of discrete (separate) devices mounted on a printed circuit board is increasingly giving way to the fabrication of complete circuits in a single package containing a ceramic substrate. Devices, such as resistors and capacitors, as well as their electric interconnections, are formed on the substrate by screen printing.
Screen printing, as is known in the art, entails forcing a liquid mixture through a patterned screen to print the components on the substrate. The substrate is then dried and fired to harden the materials in place. The screen may comprise silk, stainless steel, or other appropriate material depending upon the device being printed. The liquid mixture, herein referred to as an "ink", comprises finely divided particles, a binder, and a liquid vehicle.
Screen printing, however, poses special problems for forming capacitors. The dielectric material for the capacitors must be in the form of finely ground particles suspended in a liquid vehicle. The electric properties of the dielectric material are not the same for the bulk and the finely ground, printed and fired conditions. Thus, there is a problem producing high dielectric constants in screen printed capacitors due to the fact that the deposited material must be finely ground particles which cannot be brought back to their original, high density.
Other difficulties arise from the binders needed to hold the articles together during firing. The material used as the binder can contribute to poor dielectric constant and high dissipation factor by reacting with the dielectric during firing. Further, the firing temperatures of the less costly electrode materials are generally lower than ideal for binders which would not detract from dielectric properties. Noble metal electrodes, which can withstand higher firing temperatures, are generally too expensive for most applications. Additionally, the firing of the dielectric material in contact with the less costly electrical conductors may produce a high temperature chemical reaction between the electrode and the dielectric because of their incorporated binders.
Yet another difficulty that must be overcome is the aging characteristics of the dielectric, i.e., change in dielectric constant on the shelf and also at elevated temperatures or at high applied voltages. For example, presently used high dielectric constant materials change in dielectric constant by approximately 22% in the temperature range of from 100.degree.C to 150.degree.C, and by approximately 33% over an applied voltage range of from 0 to 167 volts per mil thickness.
It has been difficult up until this time to overcome these difficulties simultaneously with presently available dielectric inks, particularly in finding a suitable binder material for incorporation therein.
In view of the foregoing, it is therefore an object of the present invention to provide an improved dielectric ink for thick film capacitors.
Another object of the present invention is to provide an improved binder for dielectric inks that does not deteriorate the dielectric constant of the resulting material.
A further object of the present invention is to provide an improved dielectric that is more resistant to aging effects.